Depends on what kind of site you have and what you want to use the design for.
If you imagine to be doing a lot of deign in the future and need a full time designer also after launch then I would get a freelancer who could join.
If you imagine that you are going to be doing a lot of desing up front but less afterwards I would recommend you to hire an agency up front and then find some freelancer to work on it after the launch.
The latter is normally what we do.
Help our clients in the beginning and then when they have a design platform to work from we help them finding a designer to take it further.
At that point it makes no business sense from their point of view or ours. This due to the nature of how agencies normally work.
The more ahead we can plan out time the better for us (thus cheaper rates if not fixed prize) the more ad hoc and sporadic our designers are going to be used the worse for us (so more expensive and by the hours)